Japanese Patent Laying-Open No. 2003-209969 discloses a power supply control system including a plurality of power supply stages. The power supply control system includes a plurality of power supply stages connected in parallel and supplying DC power to at least one inverter. Each power supply stage includes a battery and a boost/buck DC-DC converter.
In this power supply control system, a plurality of power supply stages are controlled in such a manner that allows a plurality of batteries included in said plurality of power supply stages respectively to be equally charged and discharged and thereby allows respective states of charge (SOC) of the batteries to be identical to each other.
In the case where the batteries have respective charge/discharge characteristics different from each other, however, if a plurality of batteries are charged/discharged equally like the power supply control system disclosed in the above-referenced Japanese Patent Laying-Open No. 2003-209969, any of the batteries reaches a discharge limit or charge limit before other batteries do. After this, the maximum discharge characteristic or charge characteristic of the power supply system as a whole cannot be derived.